Method for distributing the capacity of a transmission system in a base station network

ABSTRACT

In a system handling communication between base transceiver stations (BTS 1 -BTS 12 ) and base station controllers (BSC) of a cellular radio network the transmission capacity is composed of a predetermined number of capacity units (T 0 -T 31 ). At least one capacity unit (TCH) can be allocated to a given base station group which comprises at least two base transceiver stations. Said capacity unit is allocated to a certain base transceiver station in the base station group when the instantaneous volume of traffic handled by said base transceiver station requires temporary allocation of additional capacity, and said capacity unit is again released so as to be allocatable to the base station group when the instantaneous volume of traffic handled by said base transceiver station no longer requires temporary allocation of additional capacity.

This is a national stage of PCT application no. PCT/FI98/00821, filed onOct. 22, 1998. Priority is claimed on that application, and on patentapplication no. FI 974034, filed on Oct. 23, 1997.

The invention relates to a method defined in the preamble of claim 1 fordistributing the capacity of a transmission system in a base stationnetwork, which method makes it possible to increase the number of basestations using a given transmission system and/or improve theutilization rate of the transmission system. The invention also relatesto a transmission system applying such a method.

Communication between a base station controller (BSC) and the basetransceiver stations (BTS) controlled by it in a GSM (Global System forMobile telecommunications) network, for example, are usually arranged asfollows: Transmission is realized using bi-directionaltime-division-based 2-Mbps systems. A system includes thirty-two 64-kbpstime slots each of which can be divided into four 16-kbps partial timeslots. One (point-to-point) or several base transceiver stations may beconnected to such a system in a chain, multidrop, loop or starconfiguration. Base transceiver stations have one or moretransmitter/receiver (TRX) units which comprise eight 16-kbpsbi-directional traffic channels (TCH). For each TRX unit thetransmission system allocates in a fixed manner two time slots fortraffic channels and one 16-kbps partial time slot for TRX signalling(TRXSIG). In addition, the system reserves in a fixed manner for eachbase transceiver station one 16-kbps partial time slot for operation andmaintenance unit signalling (OMUSIG). Thus, one transmission systemsuffices for 12 TRX units. In this maximum case only a few partial timeslots are left unused; the exact quantity depends on how many basetransceiver stations the TRX units are divided into. There are alsoarrangements in which the traffic of 10 TRX units is placed in the2-Mbps system. Furthermore, there are arrangements in which some of thetransmission system time slots contain GSM traffic and some contain NMT(Nordisk MobilTelefon) traffic or the traffic or paging traffic of someother cellular radio system.

The method according to the prior art and a system embodying it aredisclosed e.g. in a Nokia Telecommunications document “TRUA Base StationTransmission Unit, Product Description”, NTC C33315002SE_B0, NokiaTelecomnunications Oy 1995-1996. FIG. 1a shows a possible base stationnetwork using a 2-Mbps transmission system. The base transceiverstations in it are chained through a cable originating from a basestation controller 101. Base transceiver station BTS1 (102) has six TRXunits sectored e.g. in such a manner that each of the three sectors hastwo TRX units. Base transceiver stations BTS2, BTS3 and BTS4 each havetwo omnidirectional TRX units. The interface unit 103 in basetransceiver station BTS1 connects in bi-directional manner time slotsT1-T12 to the radio channels of the TRX units. Placement of trafficchannels TCH in the time slots is shown in more detail in FIG. 2. Rowsin the table correspond to time slots T0-T31 and X indicates that thepartial time slot in question is unused. Similarly, base transceiverstation BTS2 reserves time slots T13-T16, BTS3 time slots T17-T20, andBTS4 time slots T21-T24.

Separate time slots must be allocated for signalling (TRXSIG) andoperation and maintenance (OMUSIG). In the exemplary case, basetransceiver station BTS1 uses for these purposes time slots T25, T26 andT27, BTS2 uses time slot T28, BTS3 time slot T29 and BTS4 time slot T30in accordance with FIG. 2.

The base station network shown in FIG. 1a has a chain topology. In FIG.1b, the base station network has a star topology as a connection isbranched from base transceiver station BTS3 to two other basetransceiver stations BTS5 and BTS6. TRX units are distributed betweenthe base transceiver stations slightly differently from FIG. 1a in orderto keep their total number the same. In FIG. 1c the base station networkhas a loop topology as a direct communications connection is providedbetween base transceiver station BTS4 and the base station controllerBSC. The loop topology is used in prior-art base station networks mainlyfor securing communications as in this configuration all basetransceiver stations in the base station network have two alternativecommunications connections with the base station controller (thealternative connections are in the opposite directions of the loopformed by the base transceiver stations). FIG. 3 schematicallyillustrates a base transceiver station 300 in such a loop-configuredbase station network. Communication between the base transceiver station300 and other apparatus in the same base station network takes placethrough a transmission unit 301 (TRU). The transmission unit 301 is across-connect in which a certain branching table (not shown) determineshow the various time slots are connected straight through thetransmission unit 301 from left to right (or from right to left) andwhich time slots are connected via the lower part of the transmissionunit 301 to the base transceiver station control functions (BCF) part.Through the latter, the transmission capacity represented by the timeslots is distributed between the TRX units 302 and 303 of the basetransceiver station. In accordance with the usual practice, FIG. 3 showsthe various time slots as separate signal lines although in reality theyare transferred via the same physical connection. This example assumesthat six time slots are connected straight through the transmission unit301 (lines 304) and two time slots are connected to the base transceiverstation's TRX units 302 and 303 (lines 305 and 306).

In FIG. 3 the transmission unit 301 comprises two so-called Y-typeprotection switches 307 and 308 by means of which the system utilizesthe loop topology of the base station network. The transmission unitmonitors the so-called pilot information accompanying the signals comingfrom the different transmission directions and determines whether thetime slots used by the base transceiver station's TRX units 302 and 303should be routed via the left-hand-side path or via the right-hand-sidepath between the base transceiver station and base station controller.In FIG. 3, the transmission unit has detected that the time slotsrepresented by lines 305 and 306 should be transmitted via the leftroute, so the protection switches 307 and 308 have been set so as toconnect the base transceiver station's TRX units 302 and 303 to the leftbranches of lines 305 and 306. In case of a different measurement resultone or both of the protection switches 307 and 308 could be set into theother position indicated by the broken line, in which case the trafficin the time slot in question would be routed via the right-hand-sidepath in the base transceiver station 300. Setting of the protectionswitches 307 and 308 is realized such that a change is made in thecurrent branching table in the transmission unit 301. It is obvious thatin this context the directional terms “left”, “right” and “down” onlyrefer to the orientation shown FIG. 3 and bear no relation to the actualsituation.

A disadvantage of the present method is that the transmission systemreserves capacity for the base transceiver stations' TRX units accordingto the maximum traffic, regardless of the actual traffic situation.Thus, at times, the network operator has to pay for unnecessarytransmission capacity. A further disadvantage of the present method isthat if additional mobile communications capacity has to be built in agiven area to such an extent that the number of TRX units exceeds 12,the operator has to provide a new, even more underutilized transmissionsystem.

The object of the invention is to reduce the disadvantages mentionedabove. The method according to the invention is characterized by what isexpressed in the independent claims.

The basic idea of the method is as follows: At least part of the timeslots in the transmission system are shared by the base transceiverstations and their TRX units. A given time slot or partial time slot canat different moments be allocated to different TRX units according tothe traffic situation. Some of the traffic time slots are allocated tothe TRX units in a fixed manner and the rest are shared, or all timeslots are shared. In the latter case, too, it is preferable to allocatefixed partial time slots for TRX signalling (TRXSIG) and operation andmaintenance signalling (OMUSIG).

It is thus an advantage of the invention that the capacity of thetransmission system can be utilized more efficiently, because it canalways be directed to those TRX units, base transceiver stations andareas which have the most traffic. Compared to the current practice,more TRX units can be attached to the transmission system. This issignificant, especially in the case where the network operator has tolease the transmission connections. Consequentially, it is a furtheradvantage of the invention that as the amount of traffic increases in agiven area, the introduction of a new transmission system can bepostponed, as compared to the current practice.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail. In the description,reference will be made to the accompanying drawings wherein

FIGS. 1a-1 c are examples of known base station networks,

FIG. 2 illustrates the current usage of transmission system time slotsin a network according to FIG. 1a,

FIG. 3 shows a known base transceiver station in a loop-configured basestation network,

FIG. 4 shows the network of FIG. 1a expanded such that part of thetransmission system capacity is shared by the TRX units in accordancewith the invention,

FIG. 5 illustrates the usage of transmission system time slots inaccordance with the invention in a network according to FIG. 4,

FIG. 6 shows a network expanded such that the whole traffic capacity ofthe transmission system is shared by the TRX units in accordance withthe invention,

FIG. 7 illustrates the usage of transmission system time slots in thenetwork of FIG. 6,

FIG. 8 illustrates the principle of allocating a time slot or partialtime slot,

FIGS. 9a and 9 b show a base transceiver station applying the principleaccording to the invention,

FIG. 10 shows a base station network applying the principle according tothe invention, and

FIG. 11 shows a second base station network applying the principleaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 4 and 5 illustrate an example of the method according to theinvention. One base transceiver station BTS5 with two TRX units and onebase transceiver station BTS6 with one TRX unit employing thetransmission system have been added to the network. Similar to the priorart, some of the transmission system time slots are allocated in a fixedmanner to TRX units, but certain time slots are always shared by two TRXunits. Time slots T1-T8 (401) are allocated in a fixed manner to fourTRX units in base transceiver station BTS1, and time slots T9 and T10are allocated to one TRX unit in BTS2. Likewise, time slots T11-T16 areallocated to one TRX unit in base transceiver stations BTS3, BTS4 andBTS5, and time slots T17 and T18 are allocated to the only TRX unit inBTS6, as shown in FIG. 5. Time slots T19 and T20 are initially allocatedto TRX 4 (402) in base transceiver station BTS1 and TRX 10 in BTS3.Similarly, time slots T21 and T22 are initially allocated to TRX 6 inBTS1 and the other TRX unit (403) in BTS4. Further, time slots T23 andT24 are initially allocated to the other TRX units in base transceiverstations BTS2 and BTS5 in accordance with FIG. 5.

Let units TRX 1 and TRX 2 in base transceiver station BTS1 representsector 1, TRX 3 and TRX 4 sector 2, and TRX 5 and TRX 6 sector 3. If,for example, 8 traffic channels are not enough at a given moment insector 2, base transceiver station BTS1 reserves time slot T19 if it isfree. In this situation, base transceiver station BTS3 can use at most12 traffic channels (8+4). If 12 channels are not enough in sector 2 ofbase transceiver station BTS1, the base transceiver station alsoreserves time slot T20 if it is free. This would leave only eightchannels for base transceiver station BTS3. Likewise, BTS3 may reservetime slots T19 and T20 when they are free. So, time slots T19 and T20,as well as time slots T21-T24, are used according to the trafficsituation. Since the traffic peaks in the various cells and sectors ofcells usually do not coincide, the method described above does notsignificantly increase network congestion. Network design must allow forthe temporal distribution of traffic peaks; in an area containing officebuildings the traffic peaks occur at office hours, and in residentialareas they occur outside office hours. A base station group, in whichbase stations share initially allocated time slots, should include basestations in which the traffic peaks are not likely to coincide.

FIG. 5 also shows a possible way of locating in the transmission systemtime slots the signalling and operation and maintenance signals, markedTRXSIG and OMUSIG.

FIGS. 6 and 7 illustrate a second example of the method according to theinvention. The number of TRX units is 36, or threefold compared to thecase depicted by FIG. 1. Dynamic allocation of the transmission systemcapacity is now taken further than in the previous example. Let usassume that the rush-hour traffic peak value E_(h) is the same in eachTRX unit pair area and that E_(h)=16. Traffic volume is measured intraffic channels in use. Since instantaneous traffic volumes indifferent areas are independent of each other, the peak traffic valuesof the areas are summed squared. There are 18 TRX pairs, so the peaktraffic E_(hk) of the whole base station network is calculated from theformula

E _(hk)=16{square root over (18)}=67.9≈17.0·4

Since one time slot can have four traffic channels, a 17-time slotcapacity is enough for the traffic of the whole base station network ifthe traffic channels in all base stations are freely selectable. Theexample of FIGS. 6 and 7 assumes so. In accordance with FIG. 7, thenumber of traffic time slots is 18. All 72 traffic channels are sharedby all 36 TRX units and their 288 radio channels, so it can be said thatthe time slots are initially allocated to all base stations.

The procedure of allocating time slots or portions of time slots isdescribed below, referring to FIG. 8. In step 80 a base transceiverstation finds that it needs more capacity for the communication betweenitself and the base station controller. This find may be based on thefact that all time slots available to the base transceiver station arealready full or that a portion (say, 80%) of the available time slotsare fill, so that in the latter case the base transceiver station in away anticipates that the capacity is about to come to an end. Step 81 inFIG. 8 represents an allocation request for a time slot or part of it,sent by the base transceiver station to the base station controller inthe form of a standard message in a time slot available to the basetransceiver station, preferably in a control information time slot likethe OMUSIG or TRXSIG channel. In step 82 the base station controllerchecks the base station network's time slot allocation situation from atable that it maintains. If the base transceiver station's allocationrequest is acceptable (a suitable time slot or partial time slot isfree) the base station controller indicates to the base transceiverstation the identification of the time slot or partial time slot whichis allocated to the base transceiver station, step 83. This indicationis sent preferably in a control information time slot like the OMUSIG orTRXSIG channel. In response to said indication the base transceiverstation activates a control, step 84, which sets up a connection betweenthe allocated time slot (or partial time slot) and the downlink radiochannel in the base transceiver station's cross-connect, or transmissionunit. A more detailed discussion on the various connection types in thetransmission unit follows later on. The base transceiver station mayalso inform the base station controller, in accordance with step 85,that a connection was made so that in response to that information thebase station controller updates the allocation table such that the timeslot or partial time slot is marked allocated to the base transceiverstation in question, step 86. If the acknowledgment procedure accordingto steps 85 and 86 is not used, the allocation table is updated inconjunction with the checking of the allocation situation in step 82.

Allocation of additional capacity to a base transceiver station may alsobe initiated by the base station controller. In step 87 in FIG. 8 thebase station controller detects that a mobile station located in thearea of a given base transceiver station is about to receive a pagingmessage, i.e. a request to establish a new connection. The base stationcontroller also detects that the time slots allocated to the basetransceiver station are already full of other traffic. So the basestation controller starts the procedure according to step 82 in order toallocate a new time slot to the base transceiver station in the mannerdescribed above.

If the base station controller, when checking the allocation situationin accordance with step 82, finds that there are no suitable time slotsor partial time slots free, it will not indicate allocation of a newtime slot or partial time slot to the base transceiver station inaccordance with step 83. FIG. 8 does not show the release of theadditional capacity when the base transceiver station no longer needsit. The release may be based on a notice sent by the base transceiverstation, indicating that additional capacity allocated earlier is nomore needed. Alternatively, the base station controller may measure thetraffic in each time slot and/or partial time slot and thus detect timeslots or partial time slots that have no active traffic. If theallocation table shows that such a time slot or partial time slotbelongs to the initially allocated “shared” capacity of a base stationgroup and is now temporarily allocated to a given base transceiverstation, the base station controller may send to that base transceiverstation a deallocation notice and update the allocation table such thatthe time slot or partial time slot is again allocatable to any basetransceiver station in that base station group.

FIGS. 9a to 11 depict in more detail technical implementations used inthe transmission units of base transceiver stations to make the basestation network operate in accordance with the invention. FIGS. 9a and 9b show a base transceiver station 900 which has a transmission unit(TRU) 901 and a control functions part (BCF) 902 which distributescommunications capacity to the TRX units (not shown). The basetransceiver station 900 is allocated one time slot in a fixed manner. Inaddition, the base transceiver station 900 belongs to a base stationgroup in which the base transceiver stations are initially allocated atime slot represented by signal line 904. In FIG. 9a the basetransceiver station 900 has not reserved said time slot, so itstransmission unit 901 connects signal line 904 representing the timeslot straight through. In FIG. 9b the base transceiver station 900 hasreserved, with permission from the base station controller (not shown),the time slot represented by signal line 904. In FIG. 9b the allocationhas been made by adding to the branching table (not shown), whichcontrols the operation of the transmission unit 901, a Y-type protectionswitch 905 which connects the left branch of signal line 904 to the basestation control functions part 902. Instead of the protection switch onecould have a similar straight connection from the left branch of signalline 904 to the base station control functions part as with signal line903. If the base transceiver station depicted in FIGS. 9a and 9 bbelonged to a loop-configured base station network, it would be possibleto disclose branching tables the first of which (in the situationdepicted in FIG. 9a) would have one Y-type protection switch for signalline 903 and a straight connection through the transmission unit 901 forsignal line 904. The second branching table (in the situation depictedin FIG. 9b) of the transmission unit 901 would include two Y-typeprotection switches both of which would be used in order to produce thebest possible connection with the base station controller in the samemanner as described above in conjunction with the description of theprior art, referring to FIG. 3.

The Y-type protection switch is not the only connection type that can beused in the transmission units of base transceiver stations in order torealize the invention. FIG. 10 shows a base station network comprisingthree base transceiver stations 1001, 1002 and 1003 where each basetransceiver station is allocated in a fixed manner three of twelvepossible time slots (tripled signal lines 1004). In addition, the timeslots represented by signal lines 1005, 1006 and 1007 are initiallyallocated to the base station group comprised of the base transceiverstations in question. The transmission unit of each base transceiverstation includes three branching switches 1008, 1009 and 1010 thepositions of which determine the base station to which each of theinitially allocated time slots is connected. In FIG. 10, these timeslots are not allocated to any given base transceiver station, so allthe branching switches 1008, 1009 and 1010 are in the upper position.If, for example, the time slot represented by signal line 1007 isallocated to base transceiver station 1003, the corresponding switches1010 in base transceiver stations 1001 and 1002 are kept in the upperposition, and switch 1010 in base transceiver station 1003 is turned tothe lower position depicted by a broken line. So, in this case there isno need to introduce a totally new branching table in any of the basetransceiver stations since certain two-position switches are alreadydefined in the branching tables of the transmission units of all basetransceiver stations.

The branching switch shown in FIG. 10, which connects the inbound signalline either through the transmission unit or from the transmission unitto the control functions part, could be applied to the case depicted byFIGS. 9a and 9 b. The branching table of the base station transmissionunit of FIGS. 9a and 9 b would in that case include a branching switchwith which signal line 904 would be directed either through thetransmission unit 901 or via it to the control functions part.

FIG. 10 assumes that each base transceiver station can take additionalcapacity regardless of which initially allocated time slot is addressedto it. So, each base transceiver station can use any one of the timeslots represented by signal lines 1005, 1006 and 1007. However, basestation architecture may cause that a given base transceiver station canonly use a particular time slot as additional capacity. FIG. 11 shows abase station network in which the invention is applied in such a case.Base transceiver stations BTS1 to BTS9 form a chain-configured basestation network where each base transceiver station is allocated onetime slot (signal lines 1101) in a fixed manner. For simplicity, basetransceiver stations BTS4 to BTS7 are left out of the drawing, but theirposition and connections can be easily deduced from the rest of thedrawing. Three time slots (signal lines 1102, 1103 and 1104) areinitially allocated to a base station group which includes all the basetransceiver stations. Each base transceiver station has three branchingswitches 1105, 1106 and 1107 with one common line to the controlfunctions part 1108. This case requires the use of so-called conditionalbranching tables, i.e. if a connection is modified in the transmissionunit of a base transceiver station, the branching table in thattransmission unit has to be changed. The base station control functionspart need not know which time slot was allocated as additional capacity,because the transmission unit, controlled by the new branching table,directs the additional capacity represented by the allocated time slotto the base transceiver station always in the same way as seen from thecontrol functions part.

Above it was discussed mainly 2-Mbps connections between base stationsand base station controllers, in which traffic is divided into 32 timeslots which can be further divided into four partial time slots. Theinvention is in no way limited to systems based on these figures but theinventional idea can be applied to all systems in which thecommunication between base stations and base station controllers isbased on the allocation of time slots or similar capacity units. Forexample, other widely used data transmission rates apart from 2 Mbps are1.5 Mbps, 1 Mbps and 0.5 Mbps. If the communication between the basestations and a base station controller takes place on multiple parallelfrequency bands, the dynamic allocation method according to theinvention can be used on all frequency bands or on some of the frequencybands.

Above it was-assumed that as regards time slot allocation the basestations are equal i.e. a free, allocatable time slot is allotted to thebase station which first reserves it. Base stations may also be givenpriorities so that a given time slot may be primarily reserved to acertain base station and other base stations may reserve that time slotonly if the primary base station does not need it.

What is claimed is:
 1. A method for distributing transmission capacityin a system handling communication between base transceiver stations(BTS1-BTS12) and a base station controller (BSC) in a cellular radionetwork, wherein the transmission capacity is composed of apredetermined number of capacity units (T0-T31) which can be separatelyallocated, wherein at least one capacity unit (TCH) is allocatable to agiven base station group, which comprises at least two base transceiverstations, said method comprising the steps of allocating at least onecapacity unit to a first base transceiver station in said base stationgroup when the instantaneous volume of traffic handled by said firstbase transceiver station requires temporary allocation of additionalcapacity, and releasing said at least one capacity unit so as to beallocatable to the given base station group when the instantaneousvolume of traffic handled by said first base transceiver station nolonger requires temporary allocation of additional capacity.
 2. Themethod of claim 1, wherein said base station group includes only part ofthe base transceiver stations (TRX 4, 10; TRX 6, 12; TRX 8,14) operatingunder a given base station controller.
 3. The method of claim 1, whereinsaid base station group includes all the base transceiver stations (TRX1-36) operating under a given base station controller.
 4. The method ofclaim 1, further comprising the step of maintaining, by the base stationcontroller, an allocation table which shows the capacity unit allocationsituation.
 5. The method of claim 4, further comprising the step ofselecting, by the base station controller, a new capacity unit to beallocated (73) in response to an input which is one of the following: anallocation request (71) by a base transceiver station in conjunctionwith a new connection request (70) by a terminal; a request (72) from aswitching center for a connection to a certain terminal.
 6. The methodof claim 1, further comprising the step of permanently allocating acertain first part of the capacity units to a given base transceiverstation so that a certain second part of the capacity units (TCH) isallocatable to a given base station group.
 7. The method of claim 1,further comprising the step of allocating all capacity units (TCH) to agiven base station group.
 8. The method of claim 1, wherein the capacityunits are cyclically recurring time slots such that a time slotallocatable to a given base station group can only be allocated in itsentirety.
 9. The method of claim 1, wherein the capacity units arecyclically recurring time slots such that at least one time slotallocatable to a given base station group can be allocated in partialtime slots.
 10. A transmission system for handling communication betweenbase transceiver stations (BTS1-BTS12) and a base station controller(BSC) in a cellular radio network, wherein the transmission capacity ofsaid transmission system is composed of a predetermined number ofcapacity units which can be separately allocated, said transmissionsystem comprising at least one base station controller including meansfor indicating at least one capacity unit allocatable to a given basestation group, which comprises at least two base transceiver stations,whereby the at least one base station controller is equipped so as toallocate said capacity unit to a first base transceiver station in thebase station group when the instantaneous volume of traffic handled bysaid first base transceiver station requires temporary allocation ofadditional capacity.